Some rigorous results are presented for a first-order quantum phase transition between the dimerized state and Haldane-type state (i.e., a state similar to the ground state of the one-dimensional spin-1 Heisenberg chain) in the spin-1/2 coupled chains with nearest-neighbor and next-nearest-neighbor Heisenberg interactions. Also presented is a class of exact excited states in both phases. A partial phase diagram of the general spin-1/2 coupled chains is discussed.

• Received 6 April 1995

DOI:https://doi.org/10.1103/PhysRevB.52.12485